1. Field of the Invention
The present invention relates in general to a method and apparatus to support wellbore tubulars above a wellbore, and in particular to a sectional “spider” riser support table that may be assembled with clamping blocks that create a preload force between sections.
2. Brief Description of Related Art
A spider assembly is a support structure placed on a drilling platform for supporting casing as sections of casing are made up and lowered below the platform. A string of riser pipe, for example, may be supported by a spider assembly as additional sections of riser pipe are added to the string and lowered from the drilling rig table to the subsea wellhead. A riser is a type of casing that runs from an offshore drilling platform down to a subsea wellhead housing.
Spider assemblies have a cylindrical shape and may have a relatively large outer diameter. For example, some spider assemblies have an outer diameter of 196″. Casing is lowered through a bore in the center of the spider assembly as subsequent sections of casing are assembled, or “made up,” to the casing string. Support fixtures, such as casing support dogs, are mounted to the spider assembly to hold sections of casing in a vertical position during the running process.
It may be necessary to disassemble the spider into two or more sections for transportation or for emergency reasons. The 196″ diameter spider, for example, may too large to transport by some trucks on certain roads. An emergency condition may occur if a riser section is protruding through the bore of the spider assembly and the drilling rig must be moved to avoid a storm. It may be quicker to separate the spider and leave the riser in place rather than try to run the riser down or raise it up enough to disassemble it.
The joints that allow for assembly and disassembly of the spider may allow the spider to flex when a load, such as a heavy string of riser pipe, is suspended from the spider. Typical sectional spiders may have joints comprising a pin and finger-joints. These spiders have an axial deflection that could be greater than ½″. The deflection may be too great for some other tools located on the spider. Hydraulic actuators on the spider, for example, may need to line up precisely with the riser pipe or with other hydraulic actuators to make the joint between each subsequent section of casing. Thus deflection in the spider assembly may prevent the actuator from functioning properly.